1. Field of the Invention
The invention relates generally to imaging systems and methods, and more particularly to imaging systems and methods for probing internal structures of an object.
2. Description of the Related Art
Radiations, such as X-ray, gamma rays, infrared and visible light, as well as mechanical waves such as sonic and ultrasonic waves have been used to probe internal structures of an object. Radiography, mammographic imaging, ultrasound, Computed Tomography (CT), Positron Emitting Tomography (PET), and Magnetic Resonance Imaging (MRI) have been used widely for medical diagnostic purposes as well as in industrial applications and security checks.
When a beam of radiation or wave interacts with an internal structure of an object, part of the beam is absorbed, and part scattered. The un-scattered portion of the beam, i.e., the primary radiation, traces accurately the attenuation coefficient of the internal structure.
Earlier generations of Computed Tomography (CT) use narrow beams or fan beams of X-rays, which suffer little from scattered X-ray photons. Newer generations of CT systems use cone-beam X-rays, with a field size in the order of 10 cm, and flat-panel detector arrays to reconstruct the attenuation map of a patient. Such a setup has certain advantages over traditional CTs that utilize narrow-beams or fan-beams in that the cone-beam CT is faster and may achieve a more uniform resolution in a 3-dimensional space.
However, due to the wide field at the target, cone-beam X-ray beams are associated with large amount of scattered radiations that tend to blur the reconstructed images, wherein the images are reconstructed based on the radiation absorbed by the detectors. The radiation absorbed by the detectors include both primary and scattered radiation. In the low-energy range (˜100 kV) of X-ray photons involved in diagnostic imaging, the scatter-to-primary ratio (SPR) is on the order of 1, as compared to megavoltage X-rays, wherein typical SPR is on the order of 0.1 or smaller.
The necessity of separating primary and scattered radiation in cone-beam CTs has been noted (Jaffray, David A. et al., U.S. Patent Application Pub. No. 20030007601, “Cone-beam computerized tomography with a flat-panel imager”). It has been shown in measurements that scattered radiation may degrade the contrast-to-noise ratio (CNR) by a factor of 2 in a cone-beam CT images. There are also shading artifacts caused by scattered radiation. However, the benefits of volumetric (cone-beam) imaging warrant the effort to reduce scatter rather than going back to 2-D (fan-beam) or 1-D (pencil beam) imaging.
Swindell and Evans (Med. Phys., vol. 23, p. 63, 1996) had computed that the central axis SPR is almost linear with beam area, and is also almost linear with depth in water for a 6 MV beam. Bjarngard and Petti (Phys. Med. Biol. Vol. 33, p. 21, 1988) discovered that SPR(r,d), as a function of radius r and depth d, is a linear function of z:SPR(r,d)=Kμz,  (1)where μ is the linear attenuation coefficient for primary photons, and z=rd/(r+d), and K is a coefficient that depends on the attenuation coefficient μ. Nizin (Med. Phys. Vol. 18, p. 153) has used Eq. (1) to separate primary and scattered radiation in the case of a Co-60 beam in water.
Reducing scattered contamination has been achieved using grids in front of detectors. In an article “the influence of antiscatter grids on soft-tissue detectability in cone-beam computed tomography with flat-panel detectors” by Siewerdsen et al. (Med. Phys., vol. 31, p. 3506, 2004), it is demonstrated that this method filters out much of the scattered radiation based on the assumption that most of the scatter radiation is in directions different from those of primary photons. An analogous method of separating scattered radiation from primary radiation has been attempted over a thousand year ago, that is, an attempt to view stars in the daytime through a long tube. While some limited success has been achieved in this practice, its limitation is obvious in that the scattered day (solar) light usually still dominates the primary light from the stars even in the direction limited by the narrow tube.
Endo et al. (Med. Phys. Vol. 28, p. 469, 2001) studied the effect of scattered radiation on image noise in cone-beam CT, and the effectiveness of a focused collimator or a grid in reducing the noise.